HDU 1548 A strange lift

该题是一道典型的BFS题，如果你用DFS你就会要把所有的肯能枚举，而BFS就不需要这样。

#include<stdio.h>
#include<stdlib.h>
struct T
{
   int x, n;       
}q[424];
int BFS( int n,int k[],int start,int end )
{
   int first=0,tail=0,hash[224]={0};
   T t;
   hash[start]=1;
   t.x=start;t.n=0;
   q[first++]=t;  
   while( first>tail )
   {
      int count=q[tail].x-k[q[tail].x];
      if( (count>0)&&(hash[count]==0))
      {
         if( count==end )  return q[tail].n+1;
         hash[count]=1;
         t.x=count;
         t.n=q[tail].n+1; 
         q[first++]=t;   
      }
      count=q[tail].x+k[q[tail].x];
      if( (count<=n)&&hash[count]==0 )
      {
         if( count==end )  return q[tail].n+1;
         hash[count]=1;
         t.x=count;
         t.n=q[tail].n+1; 
         q[first++]=t;   
      }
      tail++;       
   }
   return -1;    
}
int main()
{
     int n,start,end;
     int k[224];
     while( scanf( "%d",&n ),n )
     {
         scanf( "%d%d",&start, &end );
         for( int i=1;i<=n;i++ )
            scanf( "%d",&k[i] );
         if( start==end ) printf( "0\n" );
         else
           printf( "%d\n",BFS( n,k,start,end ) );       
     }
     return 0;    
}